VOLUME 81, NUMBER 21 PHYSICAL REVIEW LETTERS 23 NOVEMBER 1998 Environment of Erbium in a-Si:H and a-SiO x :H C. Piamonteze, A. C. Iñiguez, and L. R. Tessler Instituto de Fı ´sica “Gleb Wataghin,” UNICAMP, CP 6165, 13083-970, Campinas, São Paulo, Brazil M. C. Martins Alves and H. Tolentino Laboratório Nacional de Luz Sı ´ncrotron, CP 6192, 13083-970, Campinas, São Paulo, Brazil (Received 17 June 1998) The chemical environment of Er in a-Si:H and a-SiO x :H was determined by extended x-ray absorption fine structure. Only one family of Er sites is found, coordinated on average with two to three O atoms (compared to six in Er 2 O 3 ). We devised a new model for the incorporation of Er in a-Si:H and a-SiO x :H. According to the model, Er is incorporated in the form of ErO d  1322d complexes, with d# 3. The minimum configuration energy is achieved for d  3 when the valence requirements of Er are fulfilled. The complexes are low symmetry environments that allow the Er 31 luminescent transition at 1.54 mm and make Er an acceptor in a-Si:H whereas it is donor in crystalline silicon. [S0031-9007(98)07668-6] PACS numbers: 61.43.Dq, 61.10.Ht, 73.61.Jc The study of triply ionized erbium (Er 31 ) lumines- cence in semiconductors has been an intensive area of research over the last few years mainly because of possible photonic applications [1]. The Er 31 ions ex- hibit atomiclike luminescence that arises from electronic transitions within its incomplete internal 4f shell. In particular, the 4 I 132 ! 4 I 152 transition from the first excited to the fundamental state emits photons with a wavelength of 1.54 mm. Because of the shielding pro- vided by the outer filled 5s 2 and 5p 6 shells, the transi- tion wavelength depends very weakly on the details of the host. This wavelength is especially important because it corresponds to the window of minimum attenuation in the silica based optical fibers currently used in optical communications. Silicon has been one of the most stud- ied hosts for Er, because of the enormous technological benefit that would result from a silicon based photonic technology. Erbium luminescence has been reported in crystalline, porous, and hydrogenated amorphous silicon (a-Si:H). Nevertheless, the understanding of the process of luminescence of Er in silicon is far from complete. The f -f transitions are electric dipole forbidden in the free ion. They become at least partially allowed in solid hosts where states of different angular momentum are ad- mixtured to the f wave functions. In pure float zone (FZ) crystalline Si the Er luminescence yield is relatively low, and this has been attributed to a highly symmetri- cal 12-fold coordinated Er site very similar to the Er site in ErSi 2 [2]. The addition of a co-dopant such as O, N, C, or F strongly enhances the luminescence yield [3]. In a-Si:H the Er 31 luminescence yield is high and tempera- ture quenching is smaller even without co-doping [4] when compared to crystalline Si, although O co-doping slightly enhances the luminescence yield [5]. The reason for this is not completely clear. Furthermore, Er 31 is an anomalous dopant in silicon. Early reports on crystalline silicon have found unexplained p-type doping [6], while more recent results indicated the expected n-type doping [7,8]. For unhydrogenated amorphous silicon it has been reported that many rare earths have acceptor character [9] and thus p-type doping is obtained, while for a-Si:H er- bium doping has been classified as n type [10], although in this study a detailed doping characterization was not reported. The chemical environment of Er in FZ and Czochralski (Cz) crystalline silicon has been investigated by extended x-ray absorption fine structure (EXAFS) [2,11]. The main result is that efficient Er luminescence is associated with a local environment very similar to that of Er in Er 2 O 3 , which led to the conclusion that the effect of oxygen is to provide a 6-fold coordinated, noncentrosymmetric O cage for Er that replaces the highly symmetric 12- fold coordinated Si cage. More recently, a dramatic reduction of the fraction of Er atoms in Si tetrahedral cages in Cz silicon has been associated with the reaction of Er with O during annealing and formation of Er-O complexes [12]. In Er doped a-Si:H, the only study of the local Er environment has been by emission Mössbauer spectroscopy [13]. This study concluded that the Er 31 environment in a-Si:H is similar to that in Er 2 O 3 . In this Letter we report on the determination of the Er 31 chemical environment in Er doped a-Si:H and a-SiO x :H by EXAFS. We show that the Er local environment in a-Si:H is similar to that in Er 2 O 3 only in the sense that the first Er neighbors are O atoms. The average coordination is much lower (saturating at a value of 3) and the Er-O distance is significantly shorter. Based on this we propose a model that accounts for the luminescence properties of Er in a-Si:H and also for the observed p-type doping effect of Er in a-Si:H. Erbium doped a-Si:H and a-SiO x :H samples were pre- pared in a conventional rf sputtering system following 4652 0031-9007 98  81(21)  4652(4)$15.00 © 1998 The American Physical Society